Membrane Stack Assembly for a Humidifier of a Fuel Cell System

This application claims priority to German Patent Application No. 102024114189.1, filed on May 21, 2024, the contents of which is hereby incorporated by reference in its entirety.

The invention relates to a membrane stack assembly for a humidifier in a fuel cell system, and to a humidifier with such a membrane stack assembly.

Conventional membrane stacks for humidifiers comprise membranes stacked on top of each other, each with a membrane layer that is impermeable to gas and permeable to moisture or water or water vapor. A space is formed between the individual membrane layers or membranes through which the gas can flow, thus forming a gas path. This means that a gas to be humidified can flow over one side of the membrane stack in the humidifier and a humid gas can flow over the other side. The two gases are separated from each other by the membrane layer of the respective membrane. Consequently, there is no mixing of the two gases flowing through the humidifier and the membrane stack. However, the moisture from the damper gas can pass through the membrane layer and be absorbed by the drier gas to moisten it. The moisture of the two gases thus equalizes when flowing through the membrane stack.

Such humidifiers are often used in fuel cell systems where dry cathode supply air has to be humidified before it is fed to the fuel cell. The cathode supply air must be humidified to prevent or at least delay the drying out of the polymer electrolyte membranes typically used in a fuel cell. Such drying out, which can be avoided by using a humidifier, has a particularly negative effect on the durability of polymer electrolyte membranes and the efficiency of the fuel cell.

However, when the membrane stack is used in a humidifier, the pressurized air flowing through the gas paths can cause the membrane stack to bulge in an undesirable way, especially in a stacking direction in which the individual membranes of the membrane stack are stacked on top of each other. This can cause damage and, in extreme cases, even destroy the membranes of the membrane stack.

It is therefore an object of the present invention to provide an improved membrane stack assembly comprising a membrane stack in which the aforementioned disadvantage is eliminated or at least counteracted. In particular, the aim is to create a solution that is as cost-effective as possible. This problem is solved by the subject matter of the independent claims. Preferred embodiments are the scope of the dependent claims.

The basic idea of the invention is therefore to brace a membrane stack, which is bounded in the stacking direction by two mechanically rigid end plates—such a membrane stack with end plates is hereinafter referred to as a “membrane stack assembly”—by means of a belt that is routed around the outside of the membrane stack, including the two end plates, and rests at least on the outside of the two end plates. This can at least counteract any unwanted expansion of the membrane stack and its end plates. In practice, this scenario occurs at the latest when pressurized air flows through the membrane stack assembly in a humidifier during operation and the membrane stack wants to expand in the stacking direction as a result. The belt is therefore preferably arranged on the membrane stack or the end plates in such a way that it completely prevents pressure-induced expansion of the membrane stack. In this way, the above-mentioned damage or destruction of the membranes due to said bulging can be prevented with comparatively little technical effort and thus also at low cost.

In detail, a membrane stack assembly according to the invention comprises a membrane stack having first fluid paths through which a first gas can flow and having second fluid paths through which a second gas can flow. The first and second fluid paths are separated from each other by means of membranes that are stacked on top of each other in a stacking direction, with each membrane being gas-tight and permeable to moisture.

This means that a gas to be humidified can flow over one side of the membrane and a humid gas can flow over the other side. The two gases are separated from each other by the membrane. Consequently, the two gases flowing over the membrane do not mix. However, the moisture from the damper gas can pass through the membrane and be absorbed by the drier gas to moisten it. The humidity of the two gases thus equalizes.

A spacer structure can be arranged between adjacent membranes in the stacking direction, i.e., in a respective fluid path formed by a space between these membranes, on which two adjacent membranes in the stacking direction are supported.

Furthermore, the membrane stack assembly includes a first and a second end plate, which are located opposite each other along the stacking direction and between which the membrane stack with the individual membranes is arranged. The two end plates limit the membrane stack assembly along the stacking direction. Plastic or metal, especially aluminum, can be chosen as the material for the end plates. At least one end plate can have a ribbed structure for mechanical stiffening. Furthermore, the membrane stack assembly comprises at least one belt, which rests with a first belt section on an outer side of the first end plate facing away from the membrane stack and rests with a second belt section on an outer side of the second end plate facing away from the membrane stack.

In a preferred embodiment, at least one belt can be designed and/or act as a tensioning belt, thereby exerting a prestressing force on at least one end plate and preferably on at least both end plates. By means of the belt formed as a tensioning belt, a force can already be exerted on the two end plates, which prevents or at least limits an expansion of the membrane stack, without an expansion of the membrane stack having to have already taken place-starting from a nominal state with a nominal expansion of the membrane stack in the stacking direction associated with this nominal state. This is particularly effective in counteracting unwanted expansion of the membrane stack along the stacking direction.

Preferably, at least one belt runs completely around the two end plates and the membrane stack in a closed loop. The belt is therefore completely wrapped around the membrane stack. In this way, the belt can be firmly attached to the membrane stack and, in the event of bulging, can immediately take effect as a tensioning belt to counteract the bulging.

According to a favorable further development, the membrane stack and the two end plates exhibit the geometry of a cuboid with six outer sides. The first outer side is formed by the first end plate and a second outer side by the second end plate. The belt or tensioning belt resting on the end plates thus prevents expansion along the stacking direction, which extends perpendicular to the first and second end plates. In this further development, the belt with a third belt section connecting the first belt section to the second belt section extends along a third outer side of the cuboid, which is different from the first and second outer sides and connects these two outer sides. Alternatively, the belt may extend along an edge between two outer sides of the cuboid that are different from the first and second outer sides. In both variants of this further development, the belt can be mounted particularly easily.

The belt can extend in a particularly preferred manner with a third belt section connecting the first belt section to the second belt section over a third outer side of the cuboid, which is formed by the membrane stack and connects the first outer side to the second outer side. Furthermore, the belt extends with a fourth belt section connecting the first belt section to the second belt section over a fourth outer side of the cuboid, which is formed by the membrane stack and lies opposite the third outer side, connecting the first outer side to the second outer side. When the belt or tensioning belt is arranged in this way on the cuboid, the belt pressing on the two end plates can be used to generate a uniform counter-pressure when the membrane stack expands along the stacking direction, which uniformly counteracts the expansion of the membrane stack in the stacking direction.

Preferably, the first and second outer sides of the cuboid each have the geometry of a rectangle. In this variant, the first and second belt sections each extend along a center longitudinal axis of the rectangle. By arranging the belt or the tensioning belt along the center longitudinal axis, a particularly even contact pressure is achieved on the two end plates, and unwanted lateral “slipping” of the belt from the two end plates is made more difficult.

According to another advantageous further development, two belts can also be provided which extend orthogonally to each other on the cuboid. This helps to even out the back pressure. In addition, a redundant structure is created in which, if one of the two belts fails, the other belt can still achieve the effect according to the invention.

The cuboid is particularly preferred to have a third, fourth, fifth, and sixth outer side, all of which connect the first outer side to the second outer side of the cuboid. In this variant, a third belt section of the belt connecting the first belt section and second belt section extends along a first edge formed between the third and fifth outer sides and also rests against this edge. Similarly, a fourth belt section of the belt connecting the first belt section and second belt section extends along a second edge formed between the fourth and sixth outer sides and also rests against this edge. This way, the sensitive sides of the membrane stack are protected from damage by the tensioning belt.

In another preferred embodiment, the first and second outer sides of the cuboid each have the geometry of a rectangle. In this embodiment, the first and second belt sections each extend along one of the two diagonals of the respective rectangle. This variant allows and supports the creation of a particularly even counterpressure by the two end plates in the event of expansion of the membrane stack in the stacking direction.

According to another advantageous further development, at least one belt is formed of at least two layers, preferably several layers, and has at least two or several belt layers arranged on top of one another. This can increase the mechanical strength of the belt. In particular, the back pressure generated by the belt when the membrane stack expands can be increased.

At least two layers, but preferably all layers, of at least one belt may be joined together in a materially integral manner along their entire length by means of a welded joint. This prevents individual belt layers from coming loose from the remaining belt layers. Furthermore, the mechanical strength of the belt is increased again compared to designs in which two or more belt layers are present, but these only lie loosely on top of each other or against each other, i.e., without a welded connection.

In an alternative preferred embodiment, at least two layers, preferably all layers, of at least one belt, in particular only, can be joined to one another in sections in a materially integral manner by means of a welded connection. This variant is easier to manufacture and assemble than the previously explained design with belt layers welded together and is therefore particularly cost-effective.

The welded connection in particular may be formed exclusively in the first and second belt sections of the belt. A particularly good compromise between the two previous designs in terms of manufacturing costs and the mechanical strength achieved is obtained when the welded connection explained above is provided between at least two belt layers in the first and second belt sections of the belt, but not in the third and fourth belt sections. This means that the welding of the belt layers only occurs on the insensitive outer sides of the end plates, but any damage to the edge region of the sensitive membranes of the membrane stack due to the effect of heat, caused by the welding process, is avoided.

According to another preferred embodiment, a guide can be formed on the first or/and second end plate, in which the first or second belt section of the belt is arranged. This guide simplifies the installation of the belt in the region of the two end plates and also ensures that the belt is securely fixed after installation.

According to a further advantageous development, a radius can be formed in the end plate on at least one transition of the first and/or second end plate to the membrane stack, on which the belt rests. In this way, damage to the belt at the edge formed between the respective end plate and the membrane stack is at least counteracted.

The belt is particularly preferably extended in a longitudinal direction and comprises a belt material which has fibers, in particular glass fibers, that are extended in the longitudinal direction, which in turn are embedded in a plastic matrix, preferably of a thermoplastic. This means that the plastic of the plastic matrix surrounds the fibers. A belt produced in this way has high strength, particularly in the longitudinal direction, and is also available at a commercially favorable cost.

Preferably, the longitudinal tensile strength of the belt measured in the longitudinal direction is at least five times, preferably ten times, the transverse tensile strength measured perpendicular to the longitudinal direction. In this way, the belt or tensioning belt, which is primarily stressed along the length of the membrane stack in the event of the stack bulging, is provided with the ability to exert a pre-tensioning force on the end plates in order to counteract said bulging.

The belt material is particularly functional when at least 70% by weight is formed by the fibers or glass fibers. This way, the belt can achieve an especially high tensile strength, particularly in the longitudinal direction and in relation to a direction perpendicular to the longitudinal direction.

According to a favorable modification of the membrane stack assembly according to the invention, at least one membrane, preferably each of the membranes of the membrane stack, comprises three membrane layers. Of these three membrane layers, a first membrane layer and a second membrane layer are each formed by a carrier layer. One of the three membrane layers is formed by a functional layer that is arranged between the two carrier layers. In this further development, the spacer structure is arranged on the outer side of the first carrier layer, facing away from the functional layer. The functional layer is designed to be gas-tight and moisture-permeable.

In a humidifier according to the invention, such a membrane of the membrane stack can be passed over on one side by dry air to be humidified and on the other side by moist air to humidify the air to be humidified. The air to be humidified is separated from the humid air by a membrane. Consequently, there can be no mixing of the humid air flowing through the membrane stack with the air to be humidified flowing through the membrane stack. However, the moisture from the damper gas can pass through the membrane layer and be absorbed by the drier gas to moisten it. The moisture of the two gases thus equalizes when flowing through the membrane stack.

According to an advantageous further development, a layer material of the functional layer can comprise or be, in particular expanded, polytetrafluoroethylene (ePTFE)—also known to the person skilled in the art as “Teflon.” This layer material is both impermeable to gas and permeable to moisture.

Alternatively or additionally, the carrier material of at least one carrier layer, preferably both carrier layers, may comprise or consist of polyethylene (PET) or polyphenylene sulfide (PPS). These materials give the membrane the necessary mechanical stability while taking up little space.

The invention further relates to a humidifier for a fuel cell system and for humidifying a first gas with moisture from a second gas. The humidifier comprises a housing that delimits a housing interior and a membrane stack assembly, which is arranged in the housing interior, has been previously presented and is thus in line with the invention. The advantages of the membrane stack assembly according to the invention, as explained above, are therefore transferred to the humidifier according to the invention. The humidifier comprises two gas inlets formed at a distance from each other on the housing for introducing the humid gas and the gas to be humidified into the housing interior and, furthermore, two gas outlets formed at a distance from each other on the housing for discharging the humid gas and the gas to be humidified from the housing interior.

Further important features and advantages of the invention are apparent from the dependent claims, from the drawings, and from the associated description of the figures with reference to the drawings.

It is understood that the above-mentioned features and those yet to be explained below can be used not only in the combination indicated in each case, but also in other combinations or on their own, without deviating from the scope of the present invention.

Preferred exemplary embodiments of the invention are shown in the drawings by way of example and will be explained in more detail in the following description, wherein identical reference signs refer to identical or similar or functionally identical elements.

illustrates an example of a membrane stack assemblyaccording to the invention in a perspective view. This comprises a membrane stackwith first fluid pathsthrough which a first fluid For first gas can flow and with second fluid pathsthrough which a second fluid For second gas can flow.

For clarification,shows part of the membrane stackofin a roughly schematic representation and in a side view.

The first and second fluid paths,are thus separated from each other by means of stacked, mutually spaced, gas-tight, and moisture-permeable membranesin a stacking direction S.

According to, the membrane stackhas a plurality of gas-tight and moisture-permeable membranesstacked on top of each other in a stacking direction S. The membranesare stacked on top of each other along the stacking direction S by means of a respective spacer structure, forming a respective intermediate spacethrough which a fluid or gas can flow. Along the stacking direction S, the individual spaceseach form a first or a second fluid pathor. Each of the spacer structurescan have several spacer elements, which are arranged at a distance from one another in the respective intermediate space.

Thus, moist air flowing through a respective first fluid pathcan flow over each of the membranesof the membrane stackon one side, and air to be humidified flowing through a second fluid pathcan flow over them on the other side. The humid air and the air to be humidified are separated from each other by a respective gas-tight membrane. Consequently, there is no mixing of the moist air with the air to be humidified. However, the moisture contained in the more humid air can pass through the respective moisture-permeable membraneand be absorbed by the drier air to moisten it.

shows a single membraneof the membrane stackin a greatly simplified representation and in a side view. Accordingly, the gas-tight and moisture-permeable membranecan have three membrane layers,,arranged on top of each other (not visible in). Of these three membrane layers-, a first membrane layerand a second membrane layerare each formed by a carrier layer,. A thirdof the three membrane layers-is formed by a functional layer, which is sandwiched between the two carrier layers,along the stacking direction. The functional layercomprises a gas-tight and moisture-permeable material. A layer material of the functional layerwith these properties is, for example, polytetrafluoroethylene (PTFE or ePTFE), in particular expanded polytetrafluoroethylene (PTFE or ePTFE). One of the two carrier layers,,, can be made of polyethylene, PET or polyphenylene sulfide (PPS), for example, which provides the necessary mechanical stability.

Referring again to, the membrane stack assemblyincludes first and second end plates,that face each other along the stacking direction S. The membrane stackwith the membranesis arranged between the two end plates,. The two end plates,thus delimit the membrane stack assemblyalong the stacking direction S. One of the materials of the two end plates,can be a plastic, for example polypropylene (PP), polyamide (PA), polyphthalamide (PPA), or polyphenylene sulfide (PPS), in each case with glass fiber content, or a metal, for example aluminum. For mechanical stiffening, both end plates,can each have a ribbed structureon the outside, i.e., facing away from the membrane stack.

In the example, the membrane stackand the two end plates,together have the geometric shape of a cuboidwith six outer sides-facing away from the membrane stack. In this case, a first outer sideof the cuboidis formed by the first end plateand a second outer sideof the cuboidis formed by the second end plate. The first and second outer sides,are thus opposite each other in the stacking direction S. A third, fourth, fifth, and sixth outer side-of the cuboideach connect the first outer sidewith the second outer sideand are each formed by the membrane stack. The third outer sideis opposite the fourth outer side, and the fifth outer sideis opposite the sixth outer side

According to, the membrane stack assembly includes a beltthat extends along a longitudinal direction L of the beltover the first, second, third, and fourth outer sides,,,and is in contact with these four outer sides-. The closed beltrunning around the membrane stackcomprises four belt sections-. The beltrests with a first belt sectionon the first outer sideand with a second belt sectionon the second outer sideof the cuboidand thus on the first and second end plates,. The first outer sideand the second outer sideeach have the geometry of a rectanglein a plan view along the stacking direction S. The first and second belt sections,extend in a plan view onto the first and second outer sides,along the stacking direction S, in each case along a center longitudinal axis M of the respective rectangle.

The beltextends with a third belt section, connecting the first belt sectionwith the second belt section, along a third outer sideof the cuboid, which is different from the first and second outer sides,and connects these two outer sides,. In addition, the beltextends along a fourth outer sideof the cuboid, which is different from the first and second outer sides,and connects these two outer sides,with a fourth belt sectionconnecting the first belt sectionto the second belt section. The third belt sectionis therefore on the third outer side, and the fourth belt sectionis on the fourth outer side. The fourth outer sidefaces the third outer side. Along the longitudinal direction L of the belt, the first, third, second, and fourth belt sections,,,follow one another. The beltwith the four belt sections-runs closed and completely around the two end plates,and the membrane stack.

The beltcan be designed as a tensioning beltand can also act as such and, in particular, exert a prestressing force on the end plates,in the event of an operational bulging of the membrane stack. Then, a prestressing force acting as a counterforce is exerted on the two end plates,by means of the tensioning belt, which prevents further expansion of the membrane stack in the stacking direction S.

shows the beltand the tensioning beltofin sections from above,shows them in a side view. As roughly illustrated in, the beltcan comprise a belt material that has fibers, preferably glass fibers, in particular continuous glass fibers, extending along the longitudinal direction L. These fibers or glass fibers can be embedded in a plastic matrixmade of a thermoplastic, which means that the fibersor glass fibers are surrounded by the thermoplastic. In the example, at least 70% by weight of the belt material is formed by the fibers or glass fibers. Due to the fibers or glass fibers extending in the longitudinal direction L, a longitudinal tensile strength of the beltmeasured along the longitudinal direction L is at least five times, preferably ten times, a transverse tensile strength measured perpendicular to the longitudinal direction L.

The side view ofillustrates that the belts,,can each be formed in multiple layers.shows an example of a single- or double-layer belt. This includes a first belt layerand a second belt layerarranged on the first belt layer. In variants of the example, there may also be a different number of belt layers,arranged on top of each other. The two belt layers,of beltare joined to each other along their entire length in a materially integral manner by means of a welded connection. Alternatively, however, it is also possible to join the individual belt layers,together in sections in a materially integral manner. In this case, it proves to be particularly advantageous to connect the two belt layers,to one another in a materially integral manner by means of a welded connection, at least in the first and second belt sections,of the belt, i.e., in the region of the two end plates,(see), and in particular to dispense with such a materially integral connection in the third and fourth belt sections,, so that the two belt layers,rest against each other without being firmly connected.

If a beltwith only a single belt layeris used, this can be connected in the region of the overlapping end sections in a materially integral manner by means of a welded connection.